Chris ...... Will I do?
A couple of questions first of all. Are you genuinely projecting film, or video? The latter is easier to control and gives other facilities. A film projector needs an operator and
will make a noise. In my answers below, I've assumed it's video or graphics running on a modern projector rather than film.
As a general rule, back projection is better as it's (normally) harder for actors to walk across the beam and the beam is not visible to the audience. However, as with anything that's foolproof, it does so depend on the quality of the fool.

If you are back projecting, you have a special screen that you point the projector at. Most modern graphics projectors (as used with video) have a facility that allows to to mirror image the signal so from the front it read's the right way round. If you can't back project, get the projector as close as possible to the screen. Try to aboid projecting across the auditorium at all costs.
Make sure you get a projector that will compensate for being projected on a screen at an angle as it's normally impossible to get the projector mounted at the right height. If you think about it, if the projector is up high, the top of the screen will be narrower than the bottom giving a funny shape image. The facility is called Keystone correction.
You will have to consider how much light will hit the screen from other sources. Even very powerful projectors will fade to nothing under bright stage lighting so that part of the set needs to be carefully screened from the normal lighting. Again, get as powerful projector as you can; I can't see any situation when you would have too bright an image.
Martin will probably add so more, but this should get you started. As I said at the beginning, I would think long and hard before using real film.
Robin